Your question is way too broad...
How many rigs are you building?
How important is ROI to you?
How many cards per rig?
How much do you pay for power?
Are you buying all the cards right away?
Is there a limit on how many rigs/cards you can run?
For instance...someone might only be able to run 2 systems and will be limited to 6-8 GPU's per rig.
That means they are limited to the maximum hash rate they can get from 12-16 GPU's
Should they get the cheapest RX470's possible and go for quick ROI and upgrade later?
Or go all out with 1080 ti's and have a longer ROI, but then higher per day profits and better resale?
Sorry, I am building a small mining operation.
No limit on rigs, essentially
ROI is the most important factor
Assuming 6 card standard but willing to do more or less
.15 kwh
I have enough funds to build one rig at the moment, and will build more in the near future